From the kind of people it's hiring, it's clear that Amazon intends to challenge Zynga, King.com, and other makers of social games head-on.

Amazon is already quietly publishing online games through a subsidiary, A2Z Research and Development, and is aggressively expanding in San Francisco—the home turf of Zynga, the leader of the social-gaming industry.

Asked about its social-gaming plans, an Amazon.com PR representative declined to comment.

In a little-noticed move, Amazon bought Reflexive Entertainment, a maker of so-called casual games, in 2008. But Zynga has since upended the business, replacing the casual business model of cheap or advertising-supported games with the dominant social-gaming revenue stream of today: in-game currency purchases.

Amazon continues to run the Reflexive business as a development studio in Orange County. But Business Insider has learned that Amazon is trying to staff up a studio in San Francisco—Zynga's home turf.

According to A2Z's website, that office houses employees working on digital-music efforts such as Amazon's MP3 download store.

Right now, there are just a handful of job listingsexplicitly mentioning games. But already, someveterans of Rivet Games, a high-flying games startup which folded up shop earlier this year, have landed at Amazon. And in the surest sign of big hiring plans ahead, Amazon has added recruiters with specific experience in hiring games developers.

We're also told that Amazon's been trying to hire a high-ranking executive to run a full-fledged studio.

And there's one more thing: Last month, Zynga declared itself openly in competition with Amazon Web Services with its new zCloud offering—at least for game developers looking for cloud computing. And Zynga's move to offer game-publishing services and social features to third-party developers likewise competes with Amazon's brand-new GameCircle service.

Disclosure: Jeff Bezos is an investor in Business Insider through his personal investment company Bezos Expeditions.